1. Field of the Invention
The present invention relates to a crankcase structure for an over head camshaft (OHC) type of internal combustion engine. More particularly, the present invention relates to a crankcase structure having a loop guide locking (engaging) recess and a loop-retaining member integrally formed on a crankcase of the OHC internal combustion engine.
2. Description of the Background Art
There is a known OHC internal combustion engine in which a loop guide engaging recess is formed at an outer periphery of a crankshaft-penetrated opening in a crankcase of the internal combustion engine, the crankshaft-penetrated opening and an opening in the vicinity of the loop guide engaging recess are each formed in a tumbler-like shape deformed from a circular shape, and a loop-retaining member for preventing an endless loop member from dropping from an intake and exhaust valve-driving sprocket at the times of maintenance, inspection or repair is formed in a plate provided as a body separate from the crankcase.
An example of such known OHC internal combustion engine in which a loop guide engaging recess is formed at the outer periphery of a crankshaft-penetrated opening in a crankcase of the internal combustion engine is disclosed in the Japanese Patent Laid-Open No. 2006-105132.
In the OHC internal combustion engine, as disclosed in the Japanese Patent Laid-Open No. 2006-105132, the plate as a body separate from the crankcase as well as the loop-retaining member is needed separately, whereby the number of component parts is increased.
In addition, an inner peripheral surface of an opening formed in a crankcase overlaps partially with a hollow cylindrical surface having a smaller radius, while the inner peripheral surface of the opening on the side remote from the crankshaft overlaps with a hollow cylindrical surface having a larger radius, to be in a tumbler-like shape. Therefore, stress concentration occurs in a constricted portion (a waist portion). In order to avoid this problem, the crankcase wall is made thicker, leading necessarily to undesirable increase in weight.
The present invention has been made to overcome such drawbacks. Accordingly, it is one of the objects of the present invention to provide a crankcase structure for an OHC internal combustion engine which has a small number of component parts, is simple in structure, and is light in weight.